3 Answers
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Crucio likely affects wizards, Muggles, most creatures the same.

The Cruciatus Curse causes extreme pain, and is used as a method of torturing.

“Pain,’ said Moody softly. ‘You don’t need thumbscrews or knives to torture someone if you can perform the Cruciatus Curse … that one was very popular once, too.” - Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)

When it’s cast on wizards (it’s never shown cast on Muggles), it usually causes the victim to scream and twitch on the ground as a result of the intense pain it causes them.

“Bellatrix raised her wand. ‘Crucio!’

Neville screamed, his legs drawn up to his chest so that the Death Eater holding him was momentarily holding him off the ground. The Death Eater dropped him and he fell to the floor, twitching and screaming in agony.” - Harry Potter and the Order of the Phoenix, Chapter 35 (Beyond the Veil)

Crucio works on creatures other than wizards in the same way - ‘Moody’ used it on a spider, and the spider began to twitch uncontrollably, in the same way a wizard being Crucioed would.

“Moody raised his wand again, pointed it at the spider, and muttered: ‘Crucio!’

At once, the spider’s legs bent in upon its body; it rolled over and began to twitch horribly, rocking from side to side. No sound came from it, but Harry was sure that if it could have given voice, it would have been screaming. Moody did not remove his wand, and the spider started to shudder and jerk more violently –” - Harry Potter and the Goblet of Fire, Chapter 14 (The Unforgivable Curses)

Therefore, it seems likely that the Cruciatus Curse would have the same effect on Muggles as it does on both wizards and spiders. There are some magical creatures, like dragons, that are resistant to spells, and those creatures might not be affected by Crucio in the same way, if it works on them at all. Muggles aren’t shown to be immune to spells in any way, and when Muggles have spells cast on them, the spells typically work the same way as on a wizard. The other Unforgivable Curses work on Muggles as they do on wizards - the Riddles, all Muggles, were killed using Avada Kedavra, and as is the case when it’s used on wizards, it left no damage.

“The Muggle authorities were perplexed. As far as I am aware, they do not know to this day how the Riddles died, for the Avada Kedavra Curse does not usually leave any sign of damage … the exception sits before me,’ Dumbledore added, with a nod to Harry’s scar.” - Harry Potter and the Half-Blood Prince, Chapter 17 (A Sluggish Memory)

Scrimgeour thinks it a possible danger that the Muggle Prime Minister could be put under the Imperius Curse, meaning the Imperius Curse works on Muggles as well.

“Well, we’re not,’ Scrimgeour cut in. ‘It’ll be a poor lookout for the Muggles if their Prime Minister gets put under the Imperius Curse.” - Harry Potter and the Half-Blood Prince, Chapter 1 (The Other Prime Minister)

Therefore, considering that the Cruciatus Curse has the same effect on spiders as it does on wizards, and that spells (including the Unforgivable Curses) typically affect Muggles in the same way as wizards, it’s almost certain that Crucio will affect Muggles in the same way as wizards. Furthermore, the Death Eaters have been known to have tortured Muggles, and though the Cruciatus Curse isn’t the only method of torture, it’s a popular one among the Death Eaters.

“I – I saw him torture countless Muggles and – and non-supporters of the Dark Lord.” - Harry Potter and the Goblet of Fire, Chapter 30 (The Pensieve)

It seems likely that one of the ways that the Death Eaters tortured Muggles was by using Crucio.

But some creatures are resistant to spells, likely including Crucio.

However, the Cruciatus Curse likely won’t work on all creatures - there are certain magical creatures that are resistant to spells, which likely would include Crucio. Many of the largest magical creatures, and ones with an XXXXX Ministry classification, are either explicitly stated to be resistant to spells, or are considered so dangerous to wizards that it seems unlikely that magic would work particularly well on them, since if spells were effective on them, they shouldn’t be so difficult for wizards to handle. The Erumpent’s hide repels charms, and is also specifically said to repel curses.

“Weighing up to a tonne, the Erumpent may be mistaken for a rhinoceros at a distance. It has a thick hide that repels most charms and curses, a large, sharp horn upon its nose and a long, rope-like tail.” - Fantastic Beasts and Where to Find Them

Dragons are powerful, and very difficult to subdue with magic, so it’s likely the Cruciatus Curse wouldn’t work particularly well on them - if it has any effect, it’d be a much lesser effect than usual.

“There’s a way, Harry. Don’t be tempted to try a Stunning Spell – dragons are strong and too powerfully magical to be knocked out by a single Stunner. You need about half-a-dozen wizards at a time to overcome a dragon –” - Harry Potter and the Goblet of Fire, Chapter 19 (The Hungarian Horntail)

Graphorn hide is even tougher than a dragon’s and also repels most spells, so it’s likely to be resistant to Crucio.

“Graphorn hide is even tougher than a dragon’s and repels most spells.” - Fantastic Beasts and Where to Find Them

The Manticore has skin that repels almost all charms, and although Crucio isn’t a charm, it’s likely that Manticore skin repels other types of spells as well, which may include Crucio.

“Manticore skin repels almost all known charms and the sting causes instant death.” - Fantastic Beasts and Where to Find Them

The Nundu isn’t explicitly said to be resistant to spells, but since at least a hundred skilled wizards are needed to subdue it, it’s very likely it is.

“A gigantic leopard that moves silently despite its size and whose breath causes disease virulent enough to eliminate entire villages, it has never yet been subdued by fewer than a hundred skilled wizards working together.” - Fantastic Beasts and Where to Find Them

Similar logic applies to other creatures classified as XXXXX - they’re not specifically stated as being immune to spells or magically powerful enough to resist them, but they likely are since they’re considered so dangerous to wizards - if spells were effective on them, it would be easy for a competent wizard to handle them, and they then shouldn’t have such high danger ratings.

We don't see Crucio being cast on a Muggle in any of the books. But we see Voldemort cast Avada Kedavra on a Muggle when killing the gardener, Frank Bryce, in Harry Potter and the Goblet of Fire.

"But I am not a man, Muggle," said the cold voice, barely audible now over the crackling of the flames. "I am much, much more than a man. However...why not? I will face you...Wormtail, come turn my chair around."
...

And then the chair was facing Frank, and he saw what was sitting in it. His walking stick fell to the floor with a clatter. He opened his mouth and let out a scream. He was screaming so loudly that he never heard the words the thing in the chair spoke as it raised a wand. There was a flash of green light, a rushing sound, and Frank Bryce crumpled. He was dead before he hit the floor.

And we can see Moody cast Crucio on a normal spider (not a magical animal at all) in the same book.

Moody raised his wand again, pointed it at the spider, and muttered, "Crucio!"

At once, the spider's legs bent in upon its body; it rolled over and began to twitch horribly, rocking from side to side. No sound came from it, but Harry was sure that if it could have given voice, it would have been screaming. Moody did not remove his wand, and the spider started to shudder and jerk more violently -

These two facts make me sure, that Crucio has the same effect on a Muggle as on wizards as the effect is not shown to be "magic people only" in the books.

This should be true for any magical or non magical creature in the Potterverse.
I'm not sure about gods, as there is no mention of any god-like- creature in the whole universe I know of.

It may not give Muggles seizures but it may affect Muggles differently to wizards, Newt Scamander in Fantastic Beasts and Where to Find Them says Muggles have different physiology to wizards/witches.

See, you're a Muggle. So our physiologies are subtly different.

Which presumably indicates that "magic" would harm Muggles more than wizards though to the contrary according to an article on Inverse located here:

The difference between wizards and Muggles was the presence of a magic gene, making the ability to cast spells hardly any different than the ability to dunk a basketball. That was an important distinction, because it meant that despite their abilities, wizards and witches were very much human.

Which would indicate that then such a spell as Crucio would affect Muggles in a more rigid and harsh pinching manner then Muggles but not cause seizures and instead would affect no Magi's and those of the wizarding community the same.

With only the difference being, Crucio on a Muggle would feel like a sword being stabbed through your chest, where as a wizard or witch would feel a large knife being stabbed through their chest, not as painful yet very painful still.